The invention relates generally to computed tomography (CT) imaging and more particularly, to a technique for reducing ring artifacts and image noise in images acquired via a computed tomography systems using focal spot wobble.
In a current computed tomography system, an X-ray source projects a fan-shaped or cone-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The X-ray beam passes through an object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The detector array includes detector elements, centered on a “pitch, each of which measure the intensity of transmitted radiation along a beam projected from the X-ray source to the particular detector element. The intensity of the transmitted radiation is dependent upon the attenuation of the X-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The signals are processed and reconstructed to form images which may be evaluated themselves or which may be associated to form a volume rendering or other representation of the imaged region. In a medical context, pathologies or other structures of interest may then be located or identified from the reconstructed or rendered volume.
The source and detector array in a conventional “third generation” CT system are rotated on a gantry around the object so that the angle at which the X-ray beam intersects the object changes during data acquisition. A group of X-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the X-ray source and detector. These projections are collected to form a tomographic projection set.
The acquired tomographic projection sets are typically stored in numerical form for computer processing to “reconstruct” a slice image according to reconstruction algorithms known in the art. For example, a projection set of fan beam projections may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images may be displayed on a conventional display, such as a CRT, LCD, or plasma display.
To improve spatial resolution of an X-ray CT system, the X-ray focal spot may be rapidly moved back and forward between a number of pre-determined positions during scanning. This process is commonly referred to as focal spot wobble. By interleaving the projection data from these wobbled focal spots, new projections can be obtained with higher sampling frequency, resulting in better image resolution. However, in some circumstances, consistent mis-match between the projections may create a ring artifact in the image while random mis-match may increase image noise. Such mis-matches may be present due to factors such as inaccurate air normalization, unstable X-ray focal points, patient motion, detector spectral response variation at different focal points and so forth. These errors normally are present in a non-wobble CT system as well, but due to the fact that such errors alter the entire projection smoothly, and the fact that the application of a high pass filter is done in the reconstruction process, these near constant errors are greatly suppressed in non-wobble systems. But, in a focal spot wobbling system, interleaving the near constant errors can result in high frequency errors.
It is therefore desirable to remove these high frequency errors and the resulting ring artifacts and image noise that may be present in focal spot wobbling systems without impacting the spatial resolution of such systems.